Method of and apparatus for forming volume type phase hologram

ABSTRACT

A method of and apparatus for forming a volume type phase hologram comprising the steps of, after exposure of a holographic material comprising a polymeric matrix having a radiation active substance associated therewith to an interference pattern of radiation, dipping the pattern-wise exposed holographic material in a swelling solution consisting of a first solvent which can act as a good solvent for the polymer of the matrix and a second solvent which can act as a poor solvent for the polymer and has a higher boiling point than that of the first solvent, and then pulling the swollen holographic material up from the swelling solution, whereby uniformly developed holograms having a large size and high qualities can be easily produced by a simple production process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a volume type phasehologram. More particularly, the present invention relates to a methodof stably forming a uniformly developed volume type phase hologramhaving an increased diffraction efficiency over a large area by using amonobath or single step development process in which a mixture of a goodsolvent with a lower boiling point and a poor solvent with a higherboiling point is utilized. The method of the present invention can beused in the production of recording materials, optical elements andother devices.

The present invention also relates to an apparatus for forming a volumetype phase hologram, particularly a developing apparatus thereof.

2. Description of the Related Art

As is well known in the art, holography is a photographic process andaccording to this process in which a subject to be recorded isirradiated with a good coherent radiation such as a laser beam, theradiation amplitude and phase is modulated according to the shape of thesubject, and then interference fringes of radiation reflected on ortransmitted through the subject is recorded in a holographic material toform a hologram producing an optical image of the subject. Further, theoptical image can be reproduced by again irradiating the hologram withthe radiation. Recently, hologram have been utilized in the productionof high-function and compact optical elements (HOE; Holographic OpticalElement), because various functions of the optical element can beconcentrated in a thin layer of the hologram, in addition to theutilization of holograms as a recording material. Typical examples ofthe optical elements include a reading system for bar code readers and alaser scanning system for laser printers.

Hitherto, the formation of the hologram has been carried out in atwo-step development process. Namely, as illustrated in FIG. 1, theprior art formation of the hologram comprises the steps of:

exposing a holographic material to an interference pattern of radiation,

swelling the pattern-wise exposed holographic material in a firstsolvent as a swelling solution, and

shrinking the swollen holographic material in a second solvent as ashrinking solution. The thus obtained hologram has excellent propertiessuch as a high diffraction efficiency and good stability. The two-stepdevelopment process and the formation of the hologram using this processcan be found in many patent disclosures, for example:

(1) Japanese Unexamined Patent Publication (Kokai) No. 53-15152,published on Feb. 10, 1989, and the corresponding U.S. Pat. No.4,173,474;

(2) Japanese Unexamined Patent Publication (Kokai) No. 53-15153,published on Feb. 10, 1989, and the corresponding U.S. Pat. No.4,172,724, No. 4,258,111 and No. 4,287,277;

(3) Japanese Unexamined Patent Publication (Kokai) No. 54-101343,published on Aug. 9, 1979, and the corresponding U.S. Pat. No.4,201,441;

(4) Japanese Unexamined Patent Publication (Kokai) No. 54-102140,published on Aug. 11, 1979, and the corresponding U.S. Pat. No.4,201,441;

(5) Japanese Unexamined Patent Publication (Kokai) No. 63-254485,published on Oct. 21, 1988; and

(6) Japanese Unexamined Patent Publication (Kokai) No. 63-266478,published on Nov. 2, 1988. Note, among these patent publications, thepublications (5) and (6) above were published in Japan after the filingdate of Japanese Patent Application No. 63-252501, filed on Oct. 6,1989, which is a basic application of the present application.

Among the above publications, for example, Japanese Kokai No. 53-15153concerns a hologram produced by causing a crosslinking reaction inaccordance with an interference pattern in a recording carrier composedof a water-insoluble polymer containing in the unit structure thereof anaromatic or heterocyclic ring having a reactive site which can bereplaced by a radical, and a halogen-containing compound. Thewater-insoluble polymer is preferably polyvinyl carbazole, and thehalogen-containing compound is preferably a polyhalogen compound. Usingthis recording carrier, the hologram can be formed as follows: Afterexposure of the recording carrier to an interference pattern ofradiation, the exposed recording carrier is first dipped in a firstsolvent having a swellability, i.e., a swelling solution to cause thecarrier to swell and at the same time, to remove unreactedhalogen-containing compound from the carrier. The swelling of thecarrier, which corresponds to the latent hologram pattern formed in theprevious exposure step, occurs throughout the overall thickness of thecarrier. After completion of the swelling treatment, the swollenrecording carrier is removed from the swelling solution, and then dippedin a second solvent which is a poor solvent for the recording carrierand is compatible with the first solvent, i.e., a shrinking solution.When the carrier is immersed in the shrinking solution, shrinkage of thecarrier immediately occurs as a result of replacement of the firstsolvent with the second solvent, and thus a differential refractiveindex between the exposed and unexposed areas or a distribution of therefractive index is produced in the carrier. This distribution of therefractive index relies upon the formation of microporous voids causedby a separation of the solvents during the solvent substitution.Compared with the weakly exposed areas, the strongly exposed areas willproduce less voids because of they are difficult to swell, and thus showa relatively increased refractive index. Upon completion of theseswelling and shrinking steps, the hologram is fixedly produced inaccordance with the swollen state of the carrier.

The two-step development processes described in Japanese Kokai No.53-15153 and the related literatures listed above have problems incommon. Namely, the problem caused by swelling solution remaining on theswollen carrier, just before the shrinking treatment. The inventorsfound that the amount of remaining swelling solution can have aremarkable effect on the optical properties of the resultant hologram,in addition to the degree of the swelling of the carrier and dissolutionof the water-insoluble polymer from the carrier in the swellingsolution. More particularly, if the amount of the swelling solutionadhered to the carrier surface is unacceptably high, excess amounts ofthe swelling solution remain as droplets, which will remain as traces inthe resultant hologram. Further, if lesser amounts of the swellingsolution are used, a desired diffraction efficiency cannot be obtaineddue to the reduced swelling level and subsequent weak development.

Another problem is that of the time lag from the end of the swellingtreatment to the start of the shrinking treatment. The inventors foundthat the swollen recording carrier, after removal from the swellingsolution and before immersion in the shrinking solution, can beadversely affected by atmospheric conditions such as wind andtemperature, i.e., the vaporization of the adhered swelling solution canoccur, and accordingly the resultant holograms do not have uniformproperties.

A third problem is that of the differential exposure of the pullingswollen carrier to the atmosphere. The inventors found that, when theswollen carrier is vertically pulled up from the swelling solution, anotable differential exposure of the carrier to the atmosphere occursdue to of the length of the carrier. Apparently, an upper portion of thepulling carrier is exposed to the atmosphere for a relatively long time,compared with the time of exposure of a lower portion of the carrier,and due to this longer exposure time, the development of the upperportion is weaker than that of the lower portion. The problem of thedifferences of the development strength is more severe for a recordingcarrier in which polystyrene or polyvinyl carbazole is used as thematrix polymer, in comparison with a recording carrier in whichpolyvinyl alcohol or polyvinyl pyrrolidone is used as the matrixpolymer. This is because the former requires use of the volatile organicsolvents as the good solvent, and the latter requires use of thesubstantially non-volatilizable solvents such as water, as the goodsolvent. Further, although such longitudinal differences of thedevelopment strength are negligible for small-sized holograms, they areparticularly remarkable and serious when large-sized holograms areproduced. It should be noted that, with regard to this third problem,although the value of the differences may be more or less varied, itessentially occurs in the two-step development process in which therecording carrier is removed from the developer in the course of thedevelopment and exposed to the atmosphere such as air or nitrogen gas,and is immersed again in the same or in a different developer tocomplete the development process. Therefore, to produce large-size anduniform holograms, there is a need for an improved development processin which the above problems are eliminated.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a method offorming a volume type phase hologram in which a two-step developmentstep is not used, and which can be applied to the production of a largearea and uniform hologram, and can be stably carried out by using asimplified process.

Another object of the present invention is to provide an apparatus forcarrying out the above method of forming of the volume type phasehologram, especially a developing apparatus thereof.

The inventors found that these objects can be satisfactorily realized ifa holographic material comprising a polymeric matrix having a radiationactive substance associated therewith is used as a recording carrier,and a single step development process is used in stead of the prior arttwo-step development process. The single step development process of thepresent invention is based on the use of a specific swelling solutionwhich consists of a fast vaporizable solvent and a slowly vaporizablesolvent as a developing solution, and a slow and constant pulling up ofthe swollen holographic material from the swelling solution. The term"associated herewith" used herein is intended to mean that the polymerof the matrix and the radiation active substance can be used in anydesired combinations thereof. For example, the polymer and radiationactive substance may be dissolved in a solvent to prepare a coatingsolution, or the radiation active substance may be dispersed in thepolymer to prepare a radiation-sensitive solution. Also, the term"pulling up" used herein is intended to mean that the swollenholographic material may be withdrawn from the swelling solution byusing any applicable methods including pulling up.

In one aspect of the present invention, there is provided a method offorming a volume type phase hologram comprising the steps of:

exposing a holographic material comprising a polymeric matrix having aradiation active substance associated therewith to an interferencepattern of radiation to form a latent image of a hologram; and

developing said latent image in a single step process by dipping thepattern-wise exposed holographic material in a swelling solutionconsisting of a first solvent which can act as a good solvent for thepolymer of said matrix and a second solvent which can act as a poorsolvent for said polymer and has a higher boiling point than that ofsaid first solvent, and; pulling the swollen holographic material upfrom said swelling solution to thereby form the hologram as a result ofa sequential vaporization of solvents from the holographic material.

In another aspect of the present invention, there is provided anapparatus for forming a volume type phase hologram, which apparatus isprovided with a developing apparatus comprising:

a developing chamber with a closed wall,

a container for a swelling solution disposed in a bottom wall of saidchamber,

a means for maintaining said swelling solution at a predetermined andconstant temperature,

a means for fixing an exposed holographic material to be developed, and

a guide means for said holographic material by which said material canbe dipped in said swelling solution and then pulled up from saidswelling solution at a predetermined speed, said guide means having afixing mean attached thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the formation of the hologram according tothe prior art two-step development process;

FIG. 2 is a flow diagram of the formation of the hologram according tothe method of the present invention;

FIGS. 3A to 3D are cross-sectional views showing, in sequence, amechanism of the formation of the hologram;

FIGS. 4A to 4D are cross-sectional views showing, in sequence, anothermechanism of the formation of the hologram;

FIG. 5 is a schematic diagram of a developing apparatus used in thepractice of the present invention;

FIG. 6 is a schematic diagram of a developing bath used in thedeveloping apparatus of FIG. 5; and

FIG. 7 is a schematic diagram of another developing bath used in thedeveloping apparatus of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The formation of the hologram according to the present invention ischaracterized by a single step development process using as a developera swelling solution consisting of a low-boiling good solvent and ahigh-boiling poor solvent. The single step development process, asillustrated in FIG. 2, comprises the steps of: (1) a pattern-wiseexposure of the holographic material, (2) a swelling treatment of theexposed material with a mixed solvent or swelling solution, (3) asequential vaporization of the solvents from the swollen material, and(4) a formation of the hologram.

As a first stage of the hologram formation, the pattern-wise exposure ismade by exposing a holographic material to an interference pattern ofradiation. The radiation used in this exposure is preferably a laserbeam, but other radiation sources conventionally used in the holographicprocess such as a mercury vapor lamp or xenon lamp may be used asdesired. The strength of the pattern-wise exposure may be widely varieddepending upon the desired results and other factors. The type ofexposure device is not limited, but preferably a conventional two-beaminterference exposure device is used. When forming a latent image ofhologram in the exposed holographic material.

Next, the latent image in the holographic material is developed bysubjecting the material to a swelling treatment in a mixed solvent or aswelling solution. The swelling solution is a mixture of two types ofsolvents, i.e., a low-boiling solvent which is a good solvent for thepolymer as a matrix of the holographic material such as polyvinylcarbazole, and a high-boiling solvent which is a poor solvent for thatpolymer. As a result of this treatment, the holographic material isswollen, and the extent of swelling can be controlled by selecting thetypes of the above solvents, and changing the mixing ratio of thesolvents.

After the swelling treatment is completed, the swollen holographicmaterial is slowly pulled up from the swelling solution to cause asequential vaporization of the solvents, i.e., the complete thedevelopment. The first solvent is initially vaporized, because it has alower boiling point than that of the second solvent, and thus only thesecond solvent remains as the retained swelling solution in theholographic material. An increased concentration of the second solventmeans that the matrix polymer is substantially not dissolved in theretained swelling solution and therefore, it separates out from saidswelling solution. Following to these steps, the second solvent isfinally vaporized, and voids having distribution which corresponds to apattern of the exposure radiation are formed. These voids ensure theformation of the intended hologram.

Although not shown in FIG. 2, after the pattern-wise exposure and beforethe swelling treatment, the exposed holographic material is preferablydiscolored to remove adverse constitutional components such as areaction initiator or sensitizer. This treatment effectively tostabilize the development process and prevent the formation of cracks.

In the practice of the present invention, the holographic material canbe selected from a plurality of well-known holographic materials, forexample, those used in the prior art two-step development process.Further, a combination of the polymeric matrix and radiation activesubstance can be widely varied, depending on the desired hologram andother factors.

In a preferred embodiment of the present invention, the polymer of thepolymeric matrix contains a carbazole ring in a recurring unit thereof,and the radiation active substance used in association with the polymeris a reaction initiator which can modify a solubility of the polymer insolvents upon exposure to radiation. The inventors found that asatisfactorily increased diffraction efficiency can be obtainedespecially when a holographic material based on the carbazolering-containing polymer is used.

The carbazole ring-containing polymer used in the present inventionincludes polyvinyl carbazole, vinyl carbazole-styrene copolymer, vinylcarbazole-vinylidene chloride copolymer, vinyl carbazole-acrylatecopolymer, vinyl carbazole-vinyl pyridine copolymer, halogen-substitutedpolyvinyl carbazole, nitrated polyvinyl carbazole, and related polymersand copolymers. These polymers have a molecular weight of at least100,000 or more, preferably 500,000 or more. Further, these polymers maybe used alone or in combination.

If desired, the carbazole ring-containing polymer may contain one ormore additional polymers, to improve the qualities of the resultingcoating or layer.

Further, since the carbazole ring-containing polymer generally haslittle or no sensitivity to a laser beam, it is necessary to add areaction initiator, a sensitizing dye and other additives to thepolymer. Suitable reaction initiators include, for example, polyiodidecompounds such as iodoform or carbon tetraiodide, organic peroxidecompounds such as 1,1', 4,4'-tetra (tert-butylperoxycarbonyl)benzophenone or tert-butylperoxyacetate, and basic dyes such asthioflavine T or crystal violet. Furthermore, when a visible radiationis used as an exposure source or a pattern of the visible radiation isrecorded in the holographic material, it is necessary to use asensitizing dye in combination with the reaction initiator. A suitablesensitizing dye for the polyiodide compounds is a polycyclic aromaticcompound such as tetraphenylnaphthacene, and a suitable sensitizing dyefor the organic peroxide compounds is a thiopyrrium salt or a coumarindye.

In another preferred embodiment of the present invention, theholographic material comprises a polymeric matrix having aphotopolymerizable substance which can act as the radiation activesubstance, dispersed therein

The kind of polymer of the polymeric matrix is not restricted, as beingas it satisfies the requirements of the hologram formation of thepresent invention. Suitable matrix polymers include, for example,naturally occurring polymeric materials such as gelatine, casein orstarch, cellulose derivatives such as cellulose acetate orcarboxymethylcellulose, semi-synthetic polymeric materials such asplastic materials, synthetic polymeric materials such aspolyisobutylene, polystyrene, terpene resin, polyacrylate, polyacrylicester, polymethacrylic ester, polyacrylonitrile, polyacrylamide,polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrrolidone, polyacetalresin, polyvinyl pyridine, polyvinyl carbazole, polybutadiene,polyoxymethylene, polyethyleneimine, amino resin, alkyd resin,polycarbonate resin, unsaturated polyester resin, allyl resin or epoxyresin, and natural rubbers such as chlorinated rubber or cyclizedrubber. These matrix polymers preferably have an increased molecularweight, because it is essential to the present invention that the matrixpolymers is swollen in the swelling solution used as a developer in thesubsequent development step, without being dissolved, i.e., infiniteswelling, in the solution. Note, it will be appreciated that most of theabove-listed matrix polymers are water-insoluble polymers which couldnot be used in the prior art holographic methods, especially thosemethods based on the two-step development process.

The photopolymerizable substance to be dispersed in the polymeric matrixpreferably comprises an ethylenically unsaturated bond-containingcompound, especially as a monomer, a polymerization initiator, andoptionally, a photosensitizing dye.

The ethylenically unsaturated bond-containing compounds useful in theholographic material of the present invention include, for example,acetoxymethylvinylketone, phenylvinylketone, divinylketone, maleimide,N-ethylmaleimide, N-3-acetoxypropylmaleimide, N-methylolacrylamide,N,N-methylenebisacrylamide, methyl acrylate, pentaerythritoltriacrylate, tris(acryloyloxyethyl)isocyanurate, triethyleneglycoldimethacrylate, dimethyleneglycol dimethacrylate and methylmethacrylate.

Further, useful polymerization initiators include, for example, ketonessuch as benzophenone or ethoxyacetophenone, organic peroxide compoundssuch as 3,3', 4,4',-tetra(t-butylperoxycarbonyl)benzophenone (BTTB),di-t-butyldiperoxyisophthalate, 2,2-bis(t-butylperoxy) butane,2,5-dimethyl-2,5-di(t-butylperoxy) hexane, t-butylhydroperoxide ormethylethylketone peroxide, polyhalogen compounds such as iodoform,carbon tetraiodide or carbon tetrabromide, azo compounds such asazoxystyrene or azobisisobutylonitrile, allene-ferric complexes,chloromethyltriazine, phenylglycine, and the like.

Furthermore, the photosensitizing dye used in combination with thematrix polymer and ethylenically unsaturated bond-containing compoundcan be optionally selected from well-known sensitizing dyes, taking anemission spectrum of the exposure source such as a laser intoconsideration. Suitable sensitizing dyes include, for example, acoumarine dye, a ketocoumarine dye, a thioxanthene dye, a thiopyrriumdye, a basic dye such as crystal violet or thioflavine T, a thioflavanecompound, and a condensed, polycyclic aromatic compound such astetraphenylnaphthacene.

Since the combination of the matrix polymer with the ethylenicallyunsaturated bond-containing compound and sensitizing dye is described indetail in, for example, "Synthesis and Application of PhotofunctionalPolymers", CMC, R&D Report No. 56, P88-96, these references should bereferred to in the practice of the present invention. The combinationmay be adjusted so that a difference of the solubility of the matrixpolymer in the swelling solution or developer from that of thephotopolymerizate in the same solution is further increased. This isbecause sometimes a satisfactory development cannot be obtained, eventhough the photopolymerizable substance is uniformly dispersed in thematrix polymer and a photopolymerization reaction occurs in theresulting uniform dispersion system. Of course, two or more matrixpolymers may be used in combination, if desired. Note, all of theconstitutional components of the holographic material according to thepresent invention may be used alone or in combination.

The holographic material of the present invention can be prepared, forexample, by uniformly mixing the constitutional components thereof, suchas the matrix polymer, photopolymerizable substance, and initiator,dissolving the mixture in a suitable solvent, and coating the resultantsolution onto a substrate by using a conventional coater such as a dipcoater, spin coater, roll coater or bar coater. This coating processshould be carried out with care, to ensure that the resulting coatinghas a flat and uniform surface.

The holographic material thus prepared is then exposed to aninterference pattern of radiation to form a latent image of a hologram.The exposure is preferably carried out with a laser beam in a two-beaminterference exposure device. Of course, other conventional exposuremethods and other exposure radiations may be used if desired. When theholographic material comprises a combination of the polymeric matrix andthe photopolymerizable substance, as a result of this pattern-wiseexposure, a photopolymerizate is produced in an exposed area of theholographic material, and an unreacted photopolymerizable substanceremains in an unexposed area thereof due to the absence of aphotopolymerization thereof. Eventually, a latent image corresponding tothe interference pattern of the exposure radiation, i.e., a hologramlatent image, is formed in the exposed holographic material.

After completion of the pattern-wise exposure, the exposed holographicmaterial is developed to change the latent image to a correspondingvisible image. Note, after exposure and before development, the exposedholographic is discolored with a solvent to remove adverseconstitutional components therefrom. These adverse components are, forexample, unreacted monomers, reaction or polymerization initiators,sensitizers such as dyes, and the like. Preferably, these components areremoved in the pre-development treatment, as this will stabilize andimprove the qualities of the holographic layer or coating, namely,prevent drawbacks such as clouding to white, formation of cracks andseparation of the holographic layer from the substrate. In theembodiment of the present invention in which a polymeric matrix is usedin association with a photopolymerizable substance, although theunreacted photopolymerizable substance can be dissolved out in a solventused as a swelling agent in the development process, the solvent systemsuitable for the removal of the photopolymerizable substance isdifferent from that suitable for the development. It is preferrable toseparately carry out the discoloration process and the developmentprocess, to provide a greater freedom of selection of a solvent for useas the swelling agent, since the limitation of the evaporation power inaddition to the solubility is applied to the swelling solvent in thepractice of the present invention.

Following the exposure and/or pre-development process, as previouslydescribed, the exposed holographic material is developed with a mixedsolution of low-boiling good solvent and high-boiling poor solvent in asingle step process. The mixed solution is particularly referred toherein as a "swelling solution". The mixed solution used as the swellingsolution may have widely varied compositions and mixing ratios dependingupon the specific constitutional components used, specific hologramsdesired and other factors.

In the embodiment of the present invention, in which the carbazolering-containing polymer is used in association with the reactioninitiator, preferably dichloromethane, chloroform, carbon tetrachloride,1,2-dichloroethane, 1,2-dichloroethylene, trichloroethylene, benzene,tetrahydrofuran or tetrahydropyrane is used as the low-boilinq qoodsolvent or first solvent. These solvents may be used alone or incombination. Also, preferably a paraffin hydrocarbon such as heptane,octane, nonane or decane, or an alcohol such as butanol or propanol isused as the high-boiling poor solvent or second solvent. These secondsolvents may be used alone or in combination.

Moreover, in view of the principle of the present invention, any resinnot compatibile with the carbazole ring-containing polymer but having avery different refractive index to that of the polymer, for example, anacrylate resin such as glycerol monomethacrylate may be used in stead ofthe second solvent. In this instance, although the resin used instead ofthe second solvent may remain in voids of the swollen polymer, it doesnot advesely affect the creation of a differential refractive indexsufficient to produce holograms because of the large difference of therefractive index thereof as described above. Namely, a distribution ofthe resin filled in the voids can contribute to the creation of thedifferential refractive index.

As described above, the swelling solution or developer is basicallyconstituted by the mixed solution of the low-boiling good solvent andhigh-boiling poor solvent, but if desired, any low-boiling poor solventsuch as pentane may be added to the developer, to control a swellingproperty of the entire developer to the polymer, or any solvent having arelatively higher boiling point and good swellability such as xylene maybe added to the developer, to inhibit the formation of voids.

In addition to the above-described embodiment, the same or differentswelling solutions may be used in another embodiment of the presentinvention, in which the polymeric matrix is used in association with thephotopolymerizable substance.

Namely, in the system wherein water-insoluble polymers such aspolystyrene, polycarbonate or polyvinyl carbazole are used as the matrixpolymer, a selection of good solvents or first solvents for the polymerscan be made over a relatively wide range. Low-boiling solvents having ahigh solubility to a plurality of resins such as dichloromethane,dichloroethane, chloroform or tetrahydrofuran may be used. Especially,dichloromethane is a suitable solvent for the object of the presentinvention, because it has a high solubility, is low-boiling, is notinflammable and has a relatively low toxicity. Further, any solventssuch as alcohols, ketones, esters and the like may be used as the poorsolvents for the polymer. Especially, paraffin hydrocarbons having alarge number of carbon atoms are suitable, because the carbon numberthereof can be freely changed in accordance with the boiling point ofthe good solvent used.

On the other hand, in the system wherein water-soluble polymers such aspolyvinyl alcohol or polyvinyl pyrrolidone are used as the matrixpolymer, it is necessary to use water as the good solvent for thepolymers, since there are no low-boiling organic solvent suitable as thegood solvent commercially available. Since water is difficult tovaporize due to its high boiling point and high evaporation energy, theselection of poor solvents for the polymers is restricted to a narrowrange. Namely, since the poor solvents must be those which can be freelymixed with water but are not easily vaporized compared with water, theisopropyl alcohol used as the poor solvent in the prior art two-stepdevelopment process can not be used in the present invention. Suitablepoor solvents for the polymers include, for example, N-methylformamide,N,N-dimethylacetoamide, N-methylpyrrolidone and the like. Further, aspecific means can be used to obtain a rapid vaporization of waterduring the development. For example, the development can be acceleratedif a temperature of the developer used is increased and a hot air isapplied to the developed holographic material immediately after pullingup same from the developer.

Immediately after completion of the swelling treatment in a single stepprocess, the swollen holographic material is pulled up or withdrawn fromthe developer to evaporate substantially all of the solvents in theswollen material through a vaporization of the solvents. Thevaporization of the solvents can be carried out only by pulling up theswollen material from the developer under the suitably controlledpulling speed conditions. Preferably, the swollen material is pulled upfrom the developer at a slower speed such that the first solvent isinitially vaporized, the matrix polymer separates out from the secondsolvent as a result of increase of concentration of the second solvent,and finally the second solvent is vaporized. The inventors found thatthe swollen holographic material should be continuously pulled up fromthe developer, preferably at a constant pull speed. Although the pullspeed can be widely varied, depending upon various factors, theivnentors found that the pulling speed is preferably about 0.1 to 100mm/sec, more preferably about 1 to 10 mm/sec, most preferably about 1 to3 mm/sec. To improve this vaporization of the solvents, an additionalacceleration means such as an application of hot air, as previouslydescribed, may be used in the pulling up operation. Moreover, the pulledup holographic material can be protected from the adverse influence ofthe surrounding atmosphere. This is because the inventors also foundthat the environmental conditions for the pulling holographic materialsuch as a flow of air or other gases attacking the material, or aconcentration of the surrounding vapor, can have a remarkable affect onthe optical properties of the resulting hologram and the state of thehologram layer or coating.

According to the present invention, the series of the pattern-wiseexposure, swelling and sequential vaporization steps result in anexcellent hologram in the form of voids in the holographic material, adistribution of the voids corresponding to a pattern of the interferenceradiation used in the exposure step. Surprisingly, according to thepresent invention, a uniform hologram having a high diffractionefficiency over a large hologram area can be stably and easily producedbecause of the use of a single step development, without a lowering ofthe excellent optical properties and resistance to the environment.Moreover, according to the present invention, it is possible to usewater-insoluble polymers as the matrix polymer with satisfactoryresults. Note, the use of water-insoluble polymers was avoided in theprior art, as previously described.

The mechanism of the formation of the hologram according to the presentinvention will be described hereinafter with reference to FIGS. 3A to 3Dand FIGS. 4A to 4D.

In the formation of the hologram, when the holographic material usedcomprises a polymer having a photopolymerizable substance dispersedtherein, a high diffraction efficiency cannot be obtained even if theunreacted photopolymerizable substance is selectively removed aftercompletion of the exposure. The reason why the high diffractionefficiency cannot be obtained is considered to be that the removal ofthe unreacted substance is not sufficient to reproduce recesses andprojections in the surface of the material, in conformity with adistribution of the removed and remaining substances. As is well-knownin the art, a step between the exposed and unexposed areas of theholographic material can be clearly observed, if a relatively wide thinfilm waveguide with an exposure area width of several 10 μm is producedfrom the holographic material as in the above-described instance.Nevertheless, such a step, or the recesses and projections, graduallybecomes obscure or indistinct with a reduction of the width of theexposure area. No recesses and projections could be observed forholograms having a spatial frequency of about 2000 lines/mm. Thereduction of the recesses and projections in the hologram surface isconsidered to be due to the following factors. When the surface recessesand projections are formed, the surface free energy is increased inaccordance with an increase of the surface area. On the other hand, avolume change in a lateral direction accompanies the increase of freeenergy, and therefore, when the spatial frequency is increased, a volumechange in a lateral direction occurs as a result of a large increase ofthe surface energy due to the formation of the recesses and projections,and thus the surface recesses and projections in the hologram arereduced. The above-explained mechanism of the hologram formation will bedescribed hereinafter with reference to FIGS. 3A to 3D.

In FIG. 3A, a recording carrier or holographic material comprises asubstrate 1 such as glass substrate having a holographic layer orphotosensitive layer 2 applied thereon. The holographic layer 2 has athickness of d₀ and comprises a polymer having a photopolymerizablesubstance 3 dispersed therein.

The pattern-wise exposure of the holographic material is carried out asshown in FIG. 3B. Upon the exposure of the holographic material to aninterference pattern of the predetermined radiation, as shown in FIG.3B, a photopolymerizate (not shown) is produced in an exposed area ofthe holographic layer 2, while the unreacted photopolymerizablesubstance 3 remains in an unexposed area of the layer 2 because of theabsence of photopolymerization thereof.

After to the exposure step, the exposed holographic material is treatedwith a solvent to remove the unreacted photopolymerizable substance fromthe unexposed area. As shown in FIG. 3C, the removal or solution of theunreacted photopolymerizable substance 3 and swelling of the layer 2 dueto intake of a solvent 4 occurs simultaneously in the unexposed area.

The solvent treatment is followed by drying, and for the reasonsdescribed above, and as shown in FIG. 3D, a layer thickness of theholographic layer 2 is reduced to d₁ and recesses and projections on thesurface of the layer 2 are flattened. As illustrated, the exposed areahas an optical path l₁, and the unexposed area has an optical path l₂.In this illustrated example, assuming that the photopolymerizablesubstance is completely polymerized, and thus all of thephotopolymerizate remains in the exposed area but no polymerization isinduced in the unexposed area, and therefore, the photopolymerizablesubstance can be entirely dissolved out of that area, the followingrelationship is obtained:

    optical path difference (Δnd).sub.1 =l.sub.1 -l.sub.2 =ρ·d.sub.1 ·(n.sub.1 -n.sub.0)

in which

ρdenotes a concentration of the photopolymerizable substance,

n₁ denotes a refractive index of the substance, and

n₀ denotes a refractive index of the polymer.

In contrast to the formation of the hologram described above withreference to FIGS. 3A to 3D, an increased diffraction efficiency can beobtained if the swelling and shrinking treatments are carried out duringthe developing process. The reasons for this satisfactory result arebased on the following. Namely, the holographic layer can be swollen toa substantially uniform thickness in a good solvent, without theformation of recesses and projections on a surface thereof. When theswollen layer is immersed in a poor solvent, the polymer in theholographic layer can precipitate as a result of the inter substitutionof the good and poor solvents, and the polymer is separate from thesolvent. Accordingly, microvoids are produced in the holographic layer.More particularly, in this treatment, an increased numbers of voids areformed in the area wherein the unreacted photopolymerizable substancewas dissolved out, in comparison with the area wherein thephotopolymerizable substance was polymerized and thus the polymerizatewas retained, because the formation of the recesses and projections inthe surface of the holographic layer is prevented due to theabove-described affect of the surface free energy. Since the refractiveindex of the holographic layer is entirely reduced upon formation of thevoids in the layer, a distribution of the refractive index correspondingto the interference fringes or patterns applied is produced, andaccordingly, a desired hologram can be obtained in the holographiclayer. The above-explained mechanism of the hologram formation will bedescribed hereinafter with reference to FIGS. 4A to 4D.

FIGS. 4A to 4D correspond to FIGS. 3A to 3D described above,respectively. The constitution and layer arrangement of the holographicmaterial, the exposure conditions and the like in this example are thesame as those of FIGS. 3A to 3D. An explanation of FIG. 4A is thereforeomitted to avoid duplication.

Upon exposure of the holographic material to an interference pattern ofradiation, the holographic layer 2 as shown in FIG. 4B is obtained.Following the exposure, the holographic material is swollen with a goodsolvent, and the swollen holographic layer 2 as shown in FIG. 4C isobtained. The layer 2 is swollen to a substantially uniformly increasedthickness, with less topographic features. The swollen holographicmaterial is further treated with a poor solvent as a shrinking solution,and as a result of the solvent substitution, the layer 2 (layerthickness d') having microvoids 5 is obtained as is shown in FIG. 4D.The reason why microvoids are formed in the holographic layer has beenexplained. Eventually, a hologram corresponding to the appliedinterference pattern can be obtained, and in this example, assuming thatthe optical path of the exposed area and that of the unexposed area isl₁ ' and l₂ ', respectively, the following relationship is obtained:##EQU1## in which ρ, n₁ and n₀ are as defined above.

As will be appreciated, the present invention is an improvement on theformer hologram formation process. Namely, the formation of microvoidsin the holographic layer according to the present invention can beaccomplished with a single development process by using a mixed solutionof the low-boiling good solvent and high-boiling poor solvent. Moreparticularly, the pattern-wise exposed holographic layer is swollen witha mixed solution consisting of the first solvent acting as a goodsolvent for the polymer and the second solvent acting as a poor solventfor the polymer. The extent of the swelling can be varied by controllingthe types and mixing ratios of the good and poor solvents, as well asthe bath temperatures applied. After the single swelling treatment, theholographic material is slowly pulled up from the mixed solution, andwhen a low-boiling solvent is used as the good sovlent, the good solventis first vaporized, and thus the poor solvent remains in the holographiclayer. As a result of the increased concentration of the poor solvent, asolubility of the entire solution to the polymer is increased. Thepolymer precipitates and separates from the poor solvent. Thevaporization of the poor solvent occurs during the pulling up of theholographic material, and thus traces of the vaporized poor solventbecome voids. Note, according to the development process of the presentinvention, since the hologram can be sequentially developed during thepulling up of the swollen holographic material, an important problem ofthe prior art process, i.e., differential development strength in alongitudinal direction of the material, is solved.

The formation of the hologram according to the present invention can becarried out by using any conventional apparatus, but as brieflydescribed in the SUMMARY OF THE INVENTION, it is preferably carried outby using a specific developing apparatus comprising (1) a developingchamber, (2) a container for a swelling solution, (3) a temperaturecontrolling means for controlling the temperature of the swellingsolution, (4) a means for fixing the exposed holographic material, and(5) a means for guiding the holographic material.

Preferably, the container for the swelling solution is provided with ajacket through which a heated or cooled water or other medium iscirculated to maintain the container and the contents thereof at adesired temperature.

Further, the means of guiding the holographic material preferablycomprises a stand having a rack surface, a guide box movable upwardlyand downwardly on the rack surface, and an arm mounted on the guide box,an end portion of the arm being connected to the means for fixing theholographic material.

Furthermore, the developing chamber preferably is provided with a gas orair inlet pipe and an exhaust pipe so that a dry atmosphere is createdin the chamber, and the container preferably is provided with a barrierwall in an upper end portion thereof so that, at an initial stage of thepulling, the pulled up holographic material is protected from directexposure to the atmosphere.

FIGS. 5 to 7 are schematic diagrams of a developing apparatus, and adeveloping bath thereof, useful in the practice of the presentinvention.

The developing apparatus of FIG. 5 comprises a developing chamber 10having a closed wall, and a container 11 for a swelling solution ordeveloping bath is disposed in a bottom portion of the chamber 10.Preferably, the container 11 has a double jacket structure asillustrated in FIG. 6. Namely, as shown in FIG. 6, a developer container24 for receiving a swelling solution or developer is provided with ajacket 26 through which a solution 27, for example, heated or cooledwater, is circulated.

The circulated solution 27 is maintained at a predetermined and constanttemperature by a temperature controlling mean 12, and is guided throughconduits 13 and 14 (FIG. 5). Accordingly, a swelling solution 25 isretained at a constant temperature in the container 24 during thedevelopment process. If desired, the container 11 may further comprise abarrier wall 28 or cylinder surrounding an upper end portion thereof asshown in FIG. 7. The barrier wall 28 effectively protects the pullingholographic material from direct exposure to the atmosphere such as airor gas, since this atmosphere can cause the many drawbacks previouslydescribed.

In the developing apparatus shown in FIG. 5, a holographic material 15is connected through a fixing means 16 such as holder to a guide means17. The guide means 17, as is shown in a preferred form in FIG. 5,comprises a stand 18 with a rack surface, a guide box 19 freely movableover said rack surface, and an arm 20 mounted on the guide box 19. Anend portion of the arm 20 is provided with the fixing means 16. Thedesign of the guide means 17 may be modified to improve the drivemechanism of the holographic material.

Further, in the shown developing apparatus, a gas or air inlet pipe 21with spraying holes 22 is provided in an upper portion thereof tointroduce a dried nitrogen gas or air into the chamber 10, and anexhaust pipe 23 is disposed in a bottom wall of the chamber 10, wherebya dry atmosphere is created in the chamber. 0f course, other gases maybe introduced into the chamber to produce the dry atmosphere, ifdesired.

Using the developing apparatus of FIG. 5, the pattern-wise exposedholographic material 15 can be developed as follows:

The holographic material 15, after pattern-wise exposure to aninterference radiation, is mounted through the fixing means 16 on thearm 20 of the guide means 17, and the guide box 19 is moved downward toimmerse the holographic material 15 in the swelling solution in theswelling solution in the container 11. The holographic material 15 isimmersed in the swelling solution for a time sufficient to cause thematerial 15 to swell to a predetermined swelling level. Immediatelyafter completion of the swelling treatment, the guide box 19 is slowlyelevated at a constant speed. Since the swollen holographic material isslowly pulled up from the swelling solution, the good and poor solventscontained in the material are sequentially vaporized, and thus a desiredhologram is obtained. The mechanism of the hologram formation waspreviously described.

As will be appreciated, if the described developing apparatus is used inthe practice of the present invention, the process of forming of thehologram is further improved. The guide means 19 disposed in the chamber10 enables the swollen holographic material 15 to be pulled up in adesired manner. Moreover, the barrier wall, if used, can remove excessswelling solution adhered to a surface of the holographic material, justbefore the start of pulling up of said material, as it can act as areservoir for temporarily receiving a saturated vapor of the firstvaporized good solvent, in addition to acting as a windbreaker. Sincethe remaining drops of the swelling solution on the surface of thepulled up holographic material 15 can be completely removed, anundesirable uneven development, which could not be avoided in the priorart methods, can be avoided or at least reduced. An additional advantageis that an enlarged scope of the preferred range of the pulling up speedof the holographic material is available and therefore, a uniformhologram can be produced with a high reproducibility.

In addition to these advantages, the developing apparatus of the presentinvention effectively prevents a peeling off of the holographic layerfrom the substrate, since the developing chamber 10 has a closedstructure and a dried air or nitrogen gas is introduced into the chamber10. Note, in the prior art method, such a peeling off of the holographiclayer was essentially due to a condensation of the included water, dueto the vaporization of the solvents and subsequent cooling of the layer.

Although in this embodiment the container 11 for the swelling solutionpreferably has a barrier wall, the shape of the barrier wall is notrestricted to the cylinder described above. Nevertheless, the cylinderis considered most preferable, and further preferably a height of thebarrier, i.e., length from the level of the swelling solution to a topend of the barrier is S^(1/2) or more, assuming that an area of theopened top of the barrier is S.

The present invention will be further described with reference to thefollowing examples.

EXAMPLE 1 Formation of Transmission Hologram

First, 7 g of polyvinyl carbazole having a weight average molecularweight (Mw) of 750,000 and 0.1 g of polycarbonate (Mw=40,000) weredissolved in 92 g of a mixed solution of tetrahydrofuran andmonochlorobenzene (1:1), and 0.7 g oftetra(tert-butylproxycarbonyl)benzophenone as a photosensitive agent and0.14 g of thiopyrrium were added to the solution to prepare a coatingsolution. The coating solution was spun-coated on a glass substratehaving a size of 150×150 mm to form a photsensitive layer having athickness of 6 μm.

The photosensitive layer was interference exposed to a two-beam Ar laser(488 nm), each beam having an intensity of 0.5 mW/cm², for threeminutes. The exposure amount was 80 mJ/cm², and a latent hologram of thespatial frequency of 2000 lines/mm was recorded.

Thereafter, the exposed layer was discolored with a mixed solution ofxylene and isopropanol, to remove the used photosensitive agenttherefrom, and the layer was dipped in a developer for 30 seconds afterair drying and the dipped layer then slowly pulled up. The developerused in this example is a mixed solution of 75 parts by weight of a goodsolvent and 25 parts by weight of a poor solvent, and is summarized inTable 1. Namely, the developer of this example was prepared by combininga good solvent selected from three types of solvents and a poor solventselected from six types of solvents (paraffin hydrocarbons).

                  TABLE 1                                                         ______________________________________                                                        boiling point                                                                             evaporation heat                                  solvent         (°C.)                                                                              (cal/g)                                           ______________________________________                                        (1) dichloromethane 39.8        78.9 (at b.p.)                                    (good solvent)                                                            (2) tetrahydrofuran 66          98.1 (at b.p.)                                    (good solvent)                                                            (3) 1,2-dichloroethane                                                                            83.5        77.3 (at b.p.)                                    (good solvent)                                                            (4) pentane (poor solvent)                                                                        36.1        92.6 (at 0° C.)                        (5) hexane (poor solvent)                                                                         68.7        91.8 (at 0° C.)                        (6) heptane (poor solvent)                                                                        98.4        90.6 (at 0° C.)                        (7) octane (poor solvent)                                                                         125.7       86.8 (at 25° C.)                       (8) nonane (poor solvent)                                                                         150.8       86.5 (at 25° C.)                       (9) decane (poor solvent)                                                                         174.1       86.3 (at 25° C.)                       ______________________________________                                    

The procedure of Example 1 was repeated by using different combinationsof the good and poor solvents listed in Table 1, to ascertain thepossible combinations useful in the single step development of thepresent invention. The results are summarized in Table 2.

                  TABLE 2                                                         ______________________________________                                        good      poor solvent                                                        solvent   4     5        6   7       8   9                                    ______________________________________                                        1         x     Δ  ∘                                                                     ∘                                                                         ∘                                                                     ∘                        2         x     x        x   ∘                                                                         ∘                                                                     --                                   3         x     x        x   ∘                                                                         ∘                                                                     --                                   ______________________________________                                         ∘ . . . good                                                      Δ . . . fair                                                            x . . . bad                                                              

The results of Table 2 show that the single step development is possiblewhen a difference in the boiling points of the good and poor solvents(practically, ease of evaporation of the mixed solution) is increased,and that a small difference in the boiling points will not allow a suchdevelopment to be conducted.

EXAMPLE 2 Formation of Transmission Hologram

First, 7 g of polyvinyl carbazole (Mw=750,000) and 0.1 g ofpolycarbonate (Mw=40,000) were dissolved in 92 g of a mixed solution oftetrahydrofuran and monochlorobenzene (1:1), and 0.7 of iodoform as aphotosensitive agent and 0.14 g of tetraphenylnaphthacene were added tothe solution to prepare a coating solution. The coating solution wasspun-coated on a glass substrate having a size of 150×150 mm to form aphotosensitive layer having a thickness of 6 μm.

A limited area (100×100 mm) of the photosensitive layer was interferenceexposed to a two-beam Ar laser (488 nm), each beam having an intensityof 0.5 mW/cm², for three minutes. The exposure amount was 80 mJ/cm², anda latent hologram of the spatial frequency of 1600 lines/mm was thusrecorded.

Thereafter, the exposed layer was discolored with a mixed solution ofxylene and isopropanol, to remove the used photosensitive agenttherefrom, and the layer was dipped in a developer having a temperatureadjusted to 20° C. for 30 seconds. The developer used was a mixedsolution of 70% by weight of dichloromethane and 30% by weight ofn-octane. The dipped layer was then pulled up at a speed of 2 mm/secfrom the developer, in an atmosphere of dry nitrogen gas, and a hologramhaving a diffraction efficiency of 65±5% at 488 nm was obtained.

EXAMPLE 3 Formation of Reflection Hologram

The procedure of Example 2 was repeated with the proviso that thephotsensitive layer had a thickness of 8 μm, both sides of theholographic material were exposed to an Ar laser (515 nm) to record areflection type latent hologram, and the developer was a mixed solutionof 68% by weight of dichloromethane, 20% by weight of octane, 5% byweight of xylene and 7% by weight of ethanol. A hologram having adiffraction efficiency of 70% at 515 nm was obtained.

EXAMPLE 4 Formation of Transmission Hologram

First, 100 part by weigh of vinyl carbazole and 2 parts by weight ofstyrene were cationically polymerized to produce avinylcarbazole-styrene copolymer having a weight average molecularweight of 700,000.

Using the resultant copolymer, the procedure of Example 2 was repeated,with the proviso that the developer was a mixed solution of 65% byweight of dichloromethane, 25% by weight of octane and 10% by weight ofethanol. A hologram having a diffraction efficiency of 75% at 488 nm wasobtained.

EXAMPLE 5 Formation of Transmission Hologram

This example is intended to explain the formation of the transmissionhologram by using the developing apparatus of FIG. 5.

First, 7 g of polyvinyl carbazole (Mw=750,000) and 0.1 g ofpolycarbonate (Mw=40,000) were dissolved in 92 g of the mixed solutionof tetrahydrofuran and monochlorobenzene (1:1), and 0.7 g of iodoform asa photosensitive agent and 0.14 g of tetraphenylnaphthacene were addedto the solution to prepare a coating solution. The coating solution wasspun-coated on a glass substrate having a size of 150×150 mm to form aphotosensitive layer having a thickness of 6 μm.

A limited area (100×100 mm) of the photosensitive layer was interferenceexposed to a two-beam Ar laser (488 nm), each beam having an intensityof 0.5 mW/cm², for three minutes. The exposure amount was 80 mJ/cm², anda latent hologram having a spatial frequency of 1600 lines/mm wasrecorded.

Thereafter, the exposed layer was discolored by dipping in a mixedsolution of 90% by weight of xylene and 10% by weight of isopropanol forfive minutes, to remove the used photosensitive agent therefrom. Thetemperature of the mixed solution was 28° C. After completion of thediscoloration, the discolored photosensitive layer removed from themixed solution was dried in a flow of dried nitrogen gas, for one hour.

The exposed and discolored photosensitive layer or holographic materialwas set in the developing apparatus of FIG. 5, in which the container 11had a barrier wall 28 as shown in FIG. 7, and immersed in a developerfor 30 seconds. The developer was a mixed solution of 70% by weight ofdichloromethane and 30% by weight of octane. The photosensitive layerwas swollen, and the swollen photosensitive layer was then pulled up ata speed of 2 mm/sec from the developer. The guide means 17 was used forthe pulling-up operation. A uniform hologram having a diffractionefficiency of 65±5% at 488 nm was obtained.

EXAMPLE 6 Formation of Transmission Hologram

The procedure of Example 5 was repeated, with the proviso that thebarrier wall was removed from the container 11 as illustrated in FIG. 6.A comparable hologram having a diffraction efficiency similar to that ofExample 5 was obtained, but it was observed that the hologram surfacehad a plurality of parallel lines in the pulling up direction, and anoise component of the diffracted light was increased.

EXAMPLE 7

This example is a comparative example.

The procedure of Example 5 as repeated, with the proviso that theswollen photosensitive layer was discontinuously pulled up from thedeveloper in accordance with the following recurring pattern: fiveseconds pulling up at a speed of 2 mm/sec; five seconds stop; fiveseconds pulling up at a speed of 2 mm/sec; five seconds stop; . . . Apoor hologram was obtained. Also, it was observed that the hologramsurface had a plurality of remarkable stripes, at intervals of 1 cm,perpendicular to the pulling up direction, and the diffractionefficiency was increased or reduced in the range of 30 to 65% at thesame pitches.

EXAMPLE 8 Formation of Reflection Hologram

A photosensitive solution having the following composition was prepared:

    ______________________________________                                        Polyvinyl carbazole    10     g                                               N-vinyl carbazole      1.0    g                                               Iodoform               0.5    g                                               Tetraphenylnaphthacene 0.2    g                                               Polycarbonate          0.2    g                                               Tetrahydrofuran        100    g                                               ______________________________________                                    

The photosensitive solution was spun-coated on a glass substrate havinga size of 150×150 mm to form a photosensitive layer at a dry thicknessof 10 μm.

A limited area (120×120 mm) of the photosensitive layer was exposed to atwo-beam Ar laser (488 nm), each beam having an intensity of 0.5 mW/cm²,for one minute and at an exposure amount of 60 mJ/cm². The Ar laser wasirradiated on both surfaces of the photosensitive layer to form areflection type latent hologram.

After the exposure, the photosensitive layer was discolored with a mixedsolution of xylene and isopropanol to remove the low molecular weightcomponents therefrom, and dried in air. The discolored photosensitivelayer was then immersed in a developer ofdichloromethane:isopropanol:octane (7:1:2) for 30 seconds, and pulled upfrom the developer at a speed of 2 mm/sec. The developed hologram had nosubstantial distribution of the developer drops on a surface thereof,and further, did not essentially show a difference of the developmentstrength in a longitudinal direction, but indicated a uniform andexcellent development. A reflection diffraction efficiency in theoverall hologram under the Bragg conditions was 95% or more.

EXAMPLE 9 Formation of Transmission Hologram

A photosensitive solution having the following composition was prepared:

    ______________________________________                                        Polyvinylcarbazole       10     g                                             Tris(acryloyloxyethyl)-  6.0    g                                             isocyanurate                                                                  4,4'-tetra(t-butyl-      1.0    g                                             peroxycarbonyl)benzophenone                                                   Ketocoumarin 6           0.2    g                                             Polycarbonate            0.2    g                                             Tetrahydrofuran          100    g                                             ______________________________________                                    

The photosensitive solution was spun-coated on a glass substrate havinga size of 150×150 mm to form a photosensitive layer at a dry thicknessof 6 μm.

A limited area (120 ×120 mm) of the photosensitive layer was exposed toa two-beam Ar laser (488 nm), each beam having an intensity of 0.5mW/cm², for one minute and at an exposure amount of 10 mJ/cm². The Arlaser was irradiated on the same side of the photosensitive layer toform transmission type a latent hologram having a spatial frequency of2000 lines/mm.

After exposure, the photosensitive layer was discolored with a mixedsolution of xylene, isopropanol, acetone and monochlorobenzene to removethe low molecular weight components, and dried in air. The discoloredphotosensitive layer was immersed in a developer ofdichloromethane:isopropanol:octane (71:9:20) for 30 seconds, and thenpulled up from the developer at a speed of 2 mm/sec. The developedhologram had no substantial distribution of the developer drops on asurface thereof, and no substantial difference of the developmentstrength in a longitudinal direction thereof, and showed an excellentuniformity. A diffraction efficiency in the overall hologram was 85% ormore with regard to the He-Ne laser.

EXAMPLE 10 Formation of Reflection Hologram

A photosensitive solution having the following composition was prepared:

    ______________________________________                                        Polyethylene chloride   10     g                                              Pentaerythritol triacrylate                                                                           5.0    g                                              Trichlorotriazine       1.0    g                                              Ketocoumarin            0.2    g                                              Polyvinyl chloride      0.2    g                                              Tetrahydrofuran         100    g                                              ______________________________________                                    

The photosensitive solution was spun-coated on a glass substrate havinga size of 150×150 mm to form a photosensitive layer at a dry thicknessof 6 μm.

A limited area (120×120 mm) of the photosensitive layer was exposed to atwo-beam Ar laser (488 nm), each beam having an intensity of 0.5 mW/cm²,for one minute and at an exposure amount of 20 mJ/cm². The Ar laser wasirradiated on both surfaces of the photosensitive layer to form areflection type latent hologram.

After exposure, the photosensitive layer was discolored with a mixedsolution of xylene and tetrahydrofuran to remove the low molecularweight components therefrom, and dried in air. The discoloredphotosensitive layer was immersed in a developer oftetrahydrofuran:acetoalcohol:nonane (6:3:1) for 30 seconds, and thenpulled up from the developer at a speed of 1 mm/sec. The developedhologram had no substantial distribution of the developer drops on asurface thereof and no substantial difference of the developmentstrength in a longitudinal direction thereof, and it showed an excellentuniformity. A diffraction efficiency in the overall hologram was 70% ormore.

EXAMPLE 11 Formation of Reflection Hologram

A photosensitive solution having the following composition was prepared:

    ______________________________________                                        Polyvinyl butyral       30     g                                              Acrylamide              9.0    g                                              N-methylacrylamide      1.0    g                                              N,N'-methylenebisacrylamide                                                                           1.0    g                                              Methylene blue          0.002  g                                              Triethanolamine         0.5    g                                              Ethanol                 100    g                                              ______________________________________                                    

The photosensitive solution was spun-coated on a glass substrate havingsize of 150×150 mm to form a photosensitive layer at a dry thickness of10 μm.

A limited area (120×120 mm) of the photosensitive layer was exposed to atwo-beam He-Ne laser (632.8 nm), each beam having an intensity of 0.5mW/cm², for one minutes and at an exposure amount of 50 mJ/cm². TheHe-Ne laser was irradiated on both surfaces of the photosensitive layerto form a reflection type latent hologram.

After exposure, the photosensitive layer was discolored with an aqueoussolution of methanol to remove the low molecular weight componentstherefrom, and dried in air. The discolored photosensitive layer wasimmersed in a developer of tetrahydrofuran:methanol:nonane (4:3:3) for30 seconds, and then pulled up from the developer at a speed of 1mm/sec. The developed hologram showed little distribution of thedeveloper drops on a surface thereof, but no difference of thedevelopment strength in a longitudinal direction thereof.

EXAMPLE 12 Formation of Reflection Hologram

A photosensitive solution having the following composition was prepared:

    ______________________________________                                        Polyvinyl pyrrolidone   10     g                                              Pentaerythritol triacetate                                                                            5.0    g                                              Arene-ferric complex    0.3    g                                              Polyvinyl alcohol       0.2    g                                              Water                   100    g                                              ______________________________________                                    

The photosensitive solution was spun-coated on a glass substrate havinga size of 150×150 mm to form a photosensitive layer at a dry thicknessof 10 μm.

A limited area (120×120 mm) of the photosensitive layer was exposed to atwo-beam Ar laser (488 nm), each beam having an intensity of 0.5 mW/cm²,for one minute and at an exposure amount of 50 mJ/cm². The Ar laser wasirradiated on both surfaces of the photosensitive layer to form areflection type latent hologram.

After exposure, the photosensitive layer was discolored with a mixtureof water and ethanol to remove the low molecular weight componentstherefrom, and dried in air. The discolored photosensitive layer wasimmersed in a developer of water and diacetone alcohol (60:40) at 70° C.for 30 seconds, and then pulled up from the developer, at a speed of 2mm/sec, while blowing hot air on to the layer surface to cause a rapidevaporation of water. The developed hologram show little distribution ofthe developer drops, but no difference of the development strength in alongitudinal direction thereof. A diffraction efficiency in the overallhologram was 85% or more.

We claim:
 1. A method of forming a volume type phase hologram comprisingthe steps of:exposing a holographic material comprising a polymericmatrix having a radiation active substance associated therewith to aninterference pattern of radiation to form a latent image, said polymericmatrix including a matrix polymer; and developing said latent image in asingle dipping and pulling step process including the sub-stepsofdipping the exposed holographic material in a swelling solutionincluding a first solvent and a second solvent, said matrix polymerbeing relatively more soluble in said first solvent than in said secondsolvent, said second solvent having a higher boiling point than that ofsaid first solvent, and pulling the swollen holographic material up fromsaid swelling solution so that the first solvent is initially vaporized,said matrix polymer separates out from the second solvent as a result ofan increase of concentration of the second solvent, and finally, thesecond solvent is vaporized to form voids having a distribution whichcorresponds to a pattern of said interference pattern, to thereby formthe hologram.
 2. A method according to claim 1, in which the matrixpolymer of said polymeric matrix is a polymer containing a carbazolering in a recurring unit thereof, and the radiation active substanceassociated with said polymer is a reaction initiator which can modify asolubility of said polymer in solvents upon exposure to radiation.
 3. Amethod according to claim 2, in which the carbazole ring-containingpolymer is selected from the group consisting of polyvinyl carbazole,vinyl carbazole-styrene copolymer, vinyl carbazole-vinylidene chloridecopolymer, vinyl carbazole-acrylate copolymer, vinyl carbazole-vinylpyridine copolymer, halogen-substituted polyvinyl carbazole and nitratedpolyvinyl carbazole.
 4. A method according to claim 2, in which thereaction initiator is selected from the group consisting of polyiodidecompounds, organic peroxide compounds and basic dyes.
 5. A methodaccording to claim 1, in which said polymeric matrix has dispersedtherein a photopolymerizable substance.
 6. A method according to claim5, in which the matrix polymer of said polymeric matrix is selected fromnaturally occurring polymeric materials, cellulose derivatives,semi-synthetic polymeric materials, synthetic polymeric materials andnatural rubbers.
 7. A method according to claim 5, in which thephotopolymerizable substance comprises an ethylenically unsaturatedbond-containing compound, a polymerization initiator and optionally aphotosensitizing dye.
 8. A method according to claim 7, in which saidethylenically unsaturated bond-containing compound is selected from thegroup consisting of acetoxymethylvinylketone, phenylvinylketone,divinylketone, maleimide, N-ethylmaleimide, N-3-acetoxypropylmaleimide,N-methylolacrylamide, N,N-methylenebisacrylamide, methyl acrylate,pentaerythritol triacrylate, tris(acryloyloxyethyl) isocyanurate,triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate andmethyl methacrylate.
 9. A method according to claim 7, in which saidpolymerization initiator is selected from the group consisting ofketones, organic peroxide compounds, polyhalogen compounds and azocompounds.
 10. A method according to claim 1, in which said holographicmaterial is prepared by coating a solution of constitutional componentsthereof onto a substrate.
 11. A method according to claim 1, in which anexposure is carried out with a laser beam in a two-beam interferenceexposure device.
 12. A method according to claim 1, in which, afterexposure and before development, the exposed holographic material isdiscolored to remove adverse constitutional components therefrom.
 13. Amethod according to claim 1, in which the first solvent is selected fromthe group consisting of dichlormethane, chloroform, carbontetrachloride, 1,2-dichloroethane, 1,2-dichloroethylene,trichloroethylene, benzene, tetrahydrofuran and tetrahydropyrane.
 14. Amethod according to claim 1, in which the second solvent is selectedfrom the group consisting of heptane, octane, nonane, decane, butanoland propanol.
 15. A method according to claim 1, in which the swollenholographic material is pulled up from said swelling solution at arelatively slow speed.
 16. A method according to claim 15, in which theswollen holographic material is continuously pulled up from saidswelling solution under suitably controlled pulling conditions.
 17. Amethod according to claim 16, in which pulling is carried out at apredetermined speed, and the pulled up holographic material is protectedfrom adverse influences of the surrounding atmosphere.
 18. A methodaccording to claim 17, in which the pulling up is carried at apredetermined speed of 0.1 to 100 mm/sec.